The Long Term Evolution (LTE) is the Fourth Generation (4G) wireless cellular communication technology, which supports various system bandwidths with the highest 20 MHz and can reach the peak rate of 200 Mbit/s. Compared with 3G technology, it has a shorter time delay and provides the seamless voice and multimedia services. The fast and efficient switchover is the basis to guarantee the incessant service and good user experience. The switchover is performed based on some specific parameters of the cell, such as, the measurement accuracy of the signal, the gap of the measurement and delay. The measurement period of the signal has an important influence for the switchover; if the measurement period is too long, then it may result that the switchover is not in time and cause the drop call or the uncomfortable user experience; if the measurement period is too short, then it causes the needless complexity and power consumption and the reduced measurement accuracy of the user equipment (UE).
The terminal in the LTE Release 8 needs to measure the signal power of the received Cell-specific reference signal (CRS), and then, according to the criterion of the base station configuration, it reports to the base station after meeting the criterion. The power of the received CRS signal is the reference signal received power (RSRP), which is defined as the mean value of the linear power on the resource position carrying the CRS signal in the measurement bandwidth.
In the LTE Release 8 standard, there have been some regulations for the measurement accuracy and period of the RSRP based on the CRS. There are different regulations for the measurement period of the RSRP of the CRS under the discontinuous reception (DRX) mode and the Non-Discontinuous Reception (Non-DRX) mode. The so-called Non-DRX mode is that, under the situation without configuring the measurement gap, the terminal can receive the signal and measure it at any time. Under the DRX mode, the terminal can receive the signal and measure it only under the active situation, while it usually cannot perform the measurement under the sleep situation. The DRX period configuration generally includes 10 ms, 20 ms, 32 ms, 40 ms, 64 ms, 80 ms, 128 ms, 160 ms, 256 ms, 320 ms, 512 ms, 640 ms, 1024 ms, 1280 ms, 2048 ms and 2560 ms. During one DRX period, the shortest active time is 1 ms and the longest active time is 200 ms.
In the LTE Release 10 standard, it introduces a new Channel-State Information Reference Signal (CSI-RS). The difference between the CSI-RS and the CRS includes the following several respects:
(1), the CSI-RS is periodically sent only on some subframes, and the CRS is sent on every subframe; the transmission period and the transmission subframe of the CSI-RS is shown in Table 1;
TABLE 1CSI-RS subframe configurationCSI-RSCSI-RS subframeCSI-RS subframePeriod TCSI-RSoffset ΔCSI-RSconfiguration ICSI-RS(subframe)(subframe)0-45ICSI-RS 5-1410ICSI-RS - 515-3420ICSI-RS - 1535-7440ICSI-RS - 35 75-15480ICSI-RS - 75
It can be seen from Table 1 that there are six kinds of configurations for the transmission period of the CSI-RS, 5 ms, 10 ms, 20 ms, 40 ms, 80 ms (the number of the subframes represents the period in Table 1, and the time of one subframe is 1 ms).
(2), the density of the CSI-RS in the frequency domain and the time domain (in one subframe) is far lower than the density of CRS.
(3), the CSI-RS is a UE specific reference signal, and the CRS is a cell specific reference signal.
In the conventional cellular communication network, in a certain area coverage, the Physical Cell Identifiers (PCI) of the cells are not identical; in this way, it can identify the signal of which cell is stronger through measuring the signal intensity of the CRS, so it can meet the requirement of the switchover by measuring the RSRP of the CRS. But with the development of the communication network, in the future cellular communication network, the situation that multiple cells share the PCI will appear, then it is unable to realize the efficient switchover only based on the measurement result of the RSRP of the CRS. And the CSI-RS is the UE specific, and the network side can determine which UE is required to be switched to which cell according to the intensity of the received signal of the CSI-RS reported by the UE. So the measurement of the CSI-RS is essential in the future communication network. And in the later Coordinated Multiple Point (Comp) technology, the management of the Comp measurement set also needs to be based on the RSRP measurement of the CSI-RS.
However, the RSRP measurement of the CSI-RS, including defining the measurement period and the measurement accuracy, has not been defined currently in the advanced LTE (LTE-A) standard yet; in order to realize the efficient switchover in the future evolved network, it is very necessary to define the RSRP measurement of the CSI-RS. Especially, when the terminal is in the DRX mode, because the period of CSI-RS and the period of the DRX may probably be different, the configuration based on the measurement period of the RSRP of CSI-RS is more complicated.